Percussion anvil bits

ABSTRACT

Anvil bits for use in drilling a formation under the impact action of a hammer piston reciprocable in a rotatable housing, the anvil portion of the bit having generally chordal surfaces engageable by driving members connected to the housing for transmitting rotary motion and torque from the housing to the bit.

United States Patent [191 Curington et al.

[451 Feb. 12, 1974 PERCUSSION ANVIL BITS [76] Inventors: Alfred R. Curington, 9418 Opelika St., Houston, Tex. 77055; Archer W. Kammerer, Jr., 1900 Yucca Avc., Fullerton, Calif. 92632 [22! Filed: Mar. 29, 1972 [21 Appl. No.: 239,046

[52] US. Cl 173/104, 64/23, 173/128 [51] Int. Cl. E216 3/00 [58] Field of Search... 173/104, 128, 129, 130, 132,

[5 6] References Cited UNITED STATES PATENTS 2,105,777 1/1938 Smith, Jr 173/104 X 3,311,177 3/1967 Collier et al. 173/133 2,908,152 10/1959 Anderson 64/23 3,334,693 8/1967 Badcocku 173/133 X 3,345,832 10/1967 Bottoms 64/23 Primary Examiner-Werner H. Schroeder Attorney, Agent, or Firm- Bernard Kriegel [57] ABSTRACT Anvil bits for use in drilling a formation under the impact action of a hammer piston reciprocable in a rotatable housing, the anvil portion of the bit having generally chordal surfaces engageable by driving members connected to the housing for transmitting rotary motion and torque from the housing to the bit.

14 Claims, 15 Drawing Figures PAIENTED FEB 1 2 m4 SHEET 3 OF 5 PMENTEU FEB I 2 I974 saw u or 5 PERCUSSION ANVIL BITS The present invention relates to bit devices for drilling a bore hole in a formation, and more particularly to anvil bits thatdrill the bore hole by being subjected to repeated impact blows, as by a pneumatically operated apparatus.

During the impacting of an air hammer piston against an anvil bit, the anvil bit has been rotated by the housing of the apparatus, so that the bottom face of the anvil bit covers substantially the full area of the bottom of the bore hole being drilled in a formation, torque being transmitted from the housing to the anvil bit through a key and keyway type of spline connection, in which a multiplicity of elongate keys and keyways are disposed circumferentially around the anvil bit and thee housing member associated therewith. Such key and keyway spline interconnections for transmitting torque between the housing and the anvil bit effect a substantial reduction inthe cross-sectional area of the anvil bit throughout the length of the key and keyway portion, which results in loss of the air hammer energy transmitted throughout the length of the anvil bit. This is believed to be due to the fact that the impacting of the hammer piston against the anvil bit creates longitudinal stress pulses which travel through the bit from the anvil end to the face end of the bit. These stress pulses or stress waves remain constant in magnitude until such time as they encounter a change in crosssectional area of the bit. At these changes in crosssectional area, part of the pulse is reflected back and a smaller magnitude pulse continues in the original direction. The amount of energy reflected back and transmitted through anintersection depends upon the relative areas before and after the intersection. Therefore, it is desirable to have as few cross-sectional area changes as possible in the bit, and also the ratio of areas before and after the intersection should be as near one as possible. The loss of energy in the reflected stress wave that is travelling back from the bit face to the anvil end only has a small effect on the frequency at which the air hammer runs. This is because the energy that is reflected on the bit face and gets back through all the intersections of the bit is transmitted back into the piston when it reaches the anvil end, if the piston is still in contact with the bit anvil. Ideally, this energy should be zero because the initial stress wave that reaches the bit face under ideal conditions would be transmitted to the rock face and dissipated in breaking rock. However, in actual practice, this never happens; therefore, the design of bit has some small effect on the frequency at which the hammer operates.

In addition to the above loss of energy, the key and keyway spline type of connection is relatively costly to manufacture, and is subject to excessive wear of the coengaging and readily slidable parts. The wear that does occur is uneven throughout the length of the key and keyway spline region.

I By virtue of the present invention, an anvil bit is provided which has a lesser decrease in cross-sectional area throughout its length than in prior bits, resulting in a substantial decrease in the amount of energy lost through the anvil bit as a result of reflections created by the transmitting of the longitudinal stress pulses from the upper end of the anvil bit to the lower drilling face of the anvil bit. In addition to operating more efficiently as a result of decreasing the energy loss, the

anvil bit can be manufactured at a substantially reduced cost. Wear between an anvil bit resulting from the torque transmitted to and through it is uniform, such that the torque drive portions of the anvil bit have a much longer life than prior types of drive portions, such as the key and keyway types heretofore used. Moreover, when wear does occur, the anvil bit is readily replaceable.

This invention possesses many other advantages, and has other objects which may be made more clearly apparent from a consideration of several forms in which it may be embodied. Such forms are shown in the drawings accompanying and forming part of the present specification. These forms will now be described in detail for the purpose of illustrating thee general principles of the invention; but it is to be understood that such detailed description is not to be taken in a limiting sense.

Referring to the drawings:

FIGS. Ia and 11b together constitute a longitudinal section through an apparatus containing an anvil bit embodying the invention, with parts in their relative positions in which a hammer piston has completed delivering an impact blow against its companion anvil bit, FIG. lb being a lower continuation of FIG. la;

FIGS. 2a and 2b are views similar to FIGS. la and lb,

with the hammer piston at its upper position, FIG. 2b being a lower continuation of FIG. 2a;

FIGS. 3a and 3b are views similar to FIGS. la and 1b, illustrating the relationship of the with the anvil bit off the bottom of the hole, allowing air to be circulated through the apparatus, FIG. 3b being a lower continuation of FIG. 3a;

FIG. 4 is a cross-section taken along the line 4-4 on FIG. 1b;

FIG. 5 is a cross-section taken along the line 5-5 on FIG. 1b;

FIG. 6 is an exploded isometric projection of a portion of the drive mechanism between the housing and the anvil bit of the apparatus;

FIG. 7 is a fragmentary longitudinal section through another embodiment of the lower portion of the appartus;

FIG. 8 is a cross-section taken along the line 88 on FIG. 7;

FIG. 9 is a fragmentary longitudinal section of still another embodiment of the invention;

FIG. 10 is a cross section taken along the line 10-10 on FIG. 9;

FIG. 11 contains curves showing the greater crosssectional area maintained through the anvil bit illustrated in FIGS. 1 to 6, inclusive, as compared with a key and keyway spline type of drive connection;

FIG. 12 is a cross section through another specific embodiment of the invention.

Anvil bits are illustrated in the drawings in connection with an air hammer apparatus A secured to the lower end of a string of drill pipe B by means of which the apparatus is rotated to correspondingly rotate a drill bit C used for drilling a bore hole D while the apparatus delivers repeated impact blows upon the anvil drill bit, compressed air being forced down the drill pipe for actuating the apparatus and for cleaning thev cuttings from the bottom of the hole. The apparatus includes an elongate housing structure 10 consisting of a plurality of parts. A main central section II of the housing structure has its upper end threadedly secured to an upper head 12, which, in turn, is threadedly secured to a sub 13 having a box 14 threadedly attached to the pin 15 at the lower end of an adjacent drill pipe section B. The lower end of the intermediate housing section is threadedly attached to a lower head or drive sub 16 having rotatable torque transmitting spline type connection 17 with an anvil 18 integral with the impact drill bit C of any suitable form, against which impact blows will be directed while the drill pipe string B and apparatus A are being rotated, to insure that the cut ting portions of the drill bit will cover the entire crosssectional area of the hole bottom E. The anvil l8 and bit C combination are sometimes referred to herein as an anvil bit.

The upper housing head 12 has one or a plurality of inlet passages 19 opening through its upper end, their lower ends communicating with an annular inlet passage 20 between the outer housing section 11 and a cylinder sleeve 21 integral with and depending from the housing head. This cylinder sleeve has upper inlet ports 22 and lower inlet ports 23 communicating with the annular inlet passage and adapted to be placed in communication with the cylinder space 24 within the cylinder sleeve. Below the lower inlet ports 23, the cylinder sleeve 21 carries a suitable seal ring 25 for sealing against a wall 26 of the housing section 11, which is of smaller internal diameter than the wall 27 of the housing section portion surrounding the annular inlet passage 20.

A hammer piston 28 is reciprocable within the housing structure 10 and its cylinder sleeve 21, this piston being adapted to deliver an impact blow against the upper end 29 of the anvil 18 splined to the lower head 16 and extending upwardly into the lower portion 30 of the housing section 11, this lower portion and its inner wall 26 having a substantially greater internal diameter S than the internal diameter T of the cylinder sleeve 21. In fact, the internal diameter S of the cylinder section is greater than that of the cylinder sleeve from the location of the cylinder sleeve seal ring 25 to the upper end of the lower head 16. An elongate circumferential internal groove 31 in the housing wall will function as an annular exhaust passage 32, as described hereinbelow.

The piston 28 has an enlarged outside diameter portion 33 reciprocable along the greater diameter wall 26 of the housing structure, the lower part 34 of this piston portion having a reduced external diameter so as to clear a limit ring 35 fitting within an internal circumferential groove 36 in the housing section, and on which a downwardly facing piston shoulder 37 is adapted to rest when the drill bit has been removed. When the drill bit C is in engagement with the bottom of the bore hole, it is held thereagainst by the lower end 38 of the lower head bearing against an upwardly facing drill bit or anvil shoulder 39.

The hammer piston 28 has a longitudinal impact passage 42 opening through its upper end and having a lower lateral branch 43 communicating with the annular space or passage 44 between an upper, smaller diameter piston portion 45 and the enlarged diameter wall 26 of the housing section below the housing sleeve 21. The lower end of this annular passage 44 is adapted to communicate with the annular exhaust passage 32 which is in communication with a lateral exhaust port 46 extending from a central exhaust passage 47 in the lower portion of the piston to the periphery of the enlarged piston portion. This central passage 47 receives the upper portion of a sleeve 48 piloted within a central air exhaust passage 49 extending through the anvil, being secured to the anvil in any suitable manner, as by welding material 48a. The exhaust passage 49 extends downwardly through the anvil 18 and drill bit C, discharging from he latter against the bottom E of the hole to clean the latter and the drill bit of cuttings, conveying the cuttings upwardly around the housing 10 and the drill pipe B to the top of the bore hole.

The hammer piston or impacting member also has a longitudinal return passage 50 opening through its lower end, its upper portion communicating with a port 51 opening into an annular inlet groove 52 in the smaller diameter portion 45 of the piston adapted to communicate with thelower inlet ports 23 when the air hammer is in its lower position engaging the anvil, as

disclosed in FIGS. la and lb. At this time, the upper inlet ports 22 are closed by the piston. Suitable piston rings 53, 53a, 53b may be mounted on the piston for preventing leakage therealong. As disclosed, an upper piston ring 53 occupies a position above the upper inlet ports 22 during the drilling operation, intermediate and lower piston rings 53a, 53b straddling the lower inlet ports 51 when the hammer piston 28 engages the anvil 18.

The smaller diameter piston portion 45 has a further reduced diameter portion 54 below the lower piston ring 53b, which defines an annular air inlet passage 56 with thee cylinder sleeve 21, so that upon elevation of the piston in the housing structure and its cylinder sleeve, the annular inlet passage 56 will be placed in communication with the lower inlet ports 23 to permit compressed air to flow from the latter through the annular passage and into the lower end 43 of the impact passage 42, such compressed air then passing into the cylinder 24 above the piston 28 for the purpose of forcing the latter downwardly and strike an impact blow against the anvil 18. During the early portion of upward movement of the piston, the upper end 57 of its enlarged piston portion engages the cylinder wall 26 above the annular exhaust groove 32 to shut off communication between the impact passage 42 and exhaust port 32. Additional upward movement of the piston will then place the annular inlet passage 56 in communication with the lower inlet ports 23 to feed compressed air into the impact passage 42, as described above.

As the piston moves downwardly, the lower end 58 of the upper piston portion of smaller diameter will move across the lower inlet ports 23 to close them from communication with the annular inlet passage 20, further downward movement then moving the shut-off corner 57 into the annular exhaust passage 32, allowing the compressed air above the piston and in the impact passage 42 to exhaust through the exhaust port 46, central exhaust passage 47, inner sleeve 48, and exhaust air passage 49 to the bottom of the hole, the energy imparted to the piston 28 by the compressed air driving it downwardly to impact it against the anvil 18. Just before impact occurs, the annular inlet groove 52 is placed in communication with the lower inlet ports 23, compressed air flowing therethrough and through the return passage 50 to the lower end of the piston, such air acting on the piston to drive it back toward its upper position. A relatively short upward travel againplaces the piston portion 59 below the groove 52 across the lower inlet ports 23 to shut off communication between them and the return passage 50, the air below the piston expanding and driving the piston upwardly, once again compressing the air in the cylinder space 24 above the piston. As the return air drives the piston upwardly, its lower shoulder 37 will move above the lower end 70 of the exhaust groove 32, allowing the return air to exhaust from below the piston around the reduced diameter portion 34 into the exhaust groove 32, from where it flows through the passages 46, 47, 48 and 49 to the bottom of the bore hole-D.

It will be noted that immediately before the piston 28 impacts against the anvil 18, the shut-off comer 57 moves off the cylinder wall 26 above the exhaust groove 32, permitting the compressed air that has driven the piston downwardly to exhaust through the passages 46, .47, 48, 49 into the bottom of the hole.

Such exhaust action takes place after the piston shoulder 58 has moved below the lower inlet ports 23. Shortly before the piston impacts against the upper end of the anvil, the annular inlet groove 52 is placed in communication with the lower inlet ports 23, allowing the compressed air to flow through the return passage 50 in order to drive the piston 28 back upwardly. The shut-off corner 57 moves above the exhaust groove 31 shortly before the piston portion 59 below the annular inlet groove 52 moves across the lower inlet ports 52 to shut off air flow into the return passage 50, the air below the piston expanding and driving the hammer piston upwardly, compressing the air in the cylinder space 24 above the piston. Near the upper end off the piston stroke, the shoulder 58 moves above the lower inlet ports 23 to feed compressed air through the impact passage 42 to the upper end of the piston, the compressed air driving the piston downwardly.

It will be noted that the compressed air that drives the piston downwardly acts over the full crosssectional area S of the piston, which is the area of the enlarged piston portion 33. This piston area can be made quite large since the enlarged piston portion slides along the inner wall 26 of the housing section. There are no restrictions in the housing that can reduce the piston diameter over which the compressed air acts, such as the restriction in the housing imposed by the cylinder sleeve 21. Thus, the area S over which the compressed air is acting on the power stroke of the hammer piston is the sum of the cross-sectional area T of the smaller diameter piston plus the annular area U which is the annular area between the periphery of the smaller diameter piston portion and the periphery of the enlarged piston portion. The compressed air for returning the hammer piston acts across the annular area R between the periphery of the enlarged piston portion and the inner wall of the exhaust passage 47 in the lower portion of the piston. However, such area R need not be large, since the compressed air flowing through the return air passage 50 need only return the piston to the upper end of its stroke, compressing the air in the cylinder space 24 above the piston so that it will not impact against the cylinder head 12. However,

the energy required to compress the air is recovered when the air later expands in driving the piston downwardly on its power stroke.

It is to be noted that all of the compressed air entering the apparatus exhaust through the lower end of the bit, insuring that the bottom of the bore hole will be sub 16, as described in detail hereinbelow. The piston 28 moves downwardly to the extent limited by engagement of its lower end with the upper end of the anvil, the upper end 82 of the piston then being disposed below the upper inlet circulating ports 22. At this time,

the shut-off corner 57 at the upper portion of the en-.

larged diameter of the piston is disposed well within the exhaust groove 31. Accordingly, compressed air can flow from the drill pipe B through the sub 13 and the inlet passages 19 into the annular inlet passage 20, proceeding through the inlet circulating ports 22 into the cylinder sleeve 21, and then passingthrough the impact passage 42, 43 to the exterior of the piston, flowing through the groove 31 and into the exhaust port 46, continuing to flow into the central exhaust piston passage 47, inner sleeve 48, and through the exhaust air passage 49 for discharge from the lower end of the bit.

in FIGS. la and lb, the apparatus is illustrated with the bit C resting against the bottom E of the hole and the hammer piston 28 having just delivered its blow to the anvil 18. The air in the cylinder above the piston has exhausted through the impact passage 42 and the groove 31 into the exhaust port 46, continuing to exhaust through the passages 47, 48, 49 from the bit and against the bottom of the bore hole. The compressed air flows through the return passage 50 to the bottom 60 of the hammer piston, acting in an upward direction over the annular area R of the piston to drive it upwardly. Initially, the shut-off corner 57 moves above the exhaust groove 31 to close it from the impact passage 42, whereupon the piston59 below the annular groove shuts ofi the lower inlet ports 23, the expanding air below the piston continuing to drive upwardly and compressing the air that remains in the cylinder space 24 above the piston, providing an air cushion to prevent the. piston from impacting against the cylinder head 12. The downwardly facing piston shoulder 58 I has now moved above the lowermost ends of the lower inlet ports 23, permitting compressed air to flow from the inlet ports 23 into the annular inlet passage 56, and through the impact passage 43, 42 to the upper portion of the piston (FIGS. 2a and 2b), which then causes the compressed air to drive the piston 28 downwardly and deliver an impact blow against the anvil. Near the limit of its downstroke, the shut-off comer 57 moves into to engage a central valve seat 204 on the upper sub. The valve stem and its head are urged upwardly to place the head 203 in engagement with its companion seat 204 by a helical compression spring 205 bearing against the lower end of the valve stem and against the base 206 of the guide bore. A bleeder passage 207 is provided in the upper housing head, its upper end communicating with the guide bore 201 and its lower end with a lateral bleeder port 208, to avoid trapping of fluid in the bore and its potentialinterference with the movement of the valve head.

Compressed air pumped down through the drill pipe string B will shift the valve head 203 downwardly from engagement with its seat 204, enabling the compressed air to flow into an upper recess 209 in the upper head through which the inlet passages 19 open. Any tendency for water or other liquid to flow upwardly into the apparatus is precluded by the upward seating of the valve head 203 against its seat 204.

During the reciprocation of the piston 28 within the housing structure 10 to deliver repeated impact blows against the anvil 18, the drill pipe string B and apparatus A are being rotated while suitable downweight is imposed on the drill pipe and housing structure, the lower end 38 of the head or drive member continually bearing against the bit or anvil shoulder 39, holding it against the bottom E of the bore hole. Rotary motion is transmitted from the housing structure 10 through the spline-like connection to the anvil bit. As disclosed in FIGS. 1 to 6, this spline-like drive connection includes a plurality of circumferentially spaced segments or keys 81, each of which is disposed in an opening 300 extending through the upper portion of the drive member 16, the outer drive surfaces 301 of the keys engaging the inner wall 302 of the housing while the inner surfaces 303 of the keys engage a companion generally chordal surfaces 304 formed in the anvil 18. The outer curved wall 305 of each segment is concave in shape and forms a passage 306 with the opposed inner wall 302 of the housing. The inner wall or surface 303 of each segment is convex in shape, conforming to the companion concave surface 304 in the anvil 18, this surface being elongate and of substantially greater length than the segment 81 and forming the base of the recess or groove 307 cut or otherwise formed in the anvil.

The outer portions 308 of the convex segment surface engage companion surfaces 309 forming the sides of the opening through the drive member 16. The upper end 310 of each segment engages the upper side 311 of the opening in the drive member, while the lower end 312 of each segment engages the lower end 313 of the opening, there being upper and lower transverse grooves 314, 315 formed in each segment, to provide'communication between the space 316 between the anvil and housing above the segment and the space 317 within the housing below the segment; that is, air can bleed from the space 316 above each segment through the upper groove 314 into the passage 306 between each segment and housing, continuing through the lower groove 315 into the lower portion of the anvil recess 317. Normally, during the imposition of drilling weight on the bit by virtue of the lower end of the head 16 engaging the bit shoulder 39, circumferential grooves 318 in the drive member 16 below its openings 300 provide a labyrinth seal engaging the cylindrical periphery 319 of the anvil below its recesses 304. However, when the drill bit is picked up off bottom E with the anvil and bit dropping downwardly to the position illustrated in FIGS. 3a and 3b, the downward movement of the bit anvil being limited by engagement of its upper head with the upper ends of the segments 81, the cylindrical periphery 319 of the anvil 18 is disposed below the labyrinth seals 318, placing the recesses or grooves 304 in the anvil in communication with circumferentially spaced internal relief grooves 320 in the interior of the drive member 16 that opens through its lower end. Thus, upon elevating of the bit from the bottom of the hole E and its downward dropping, air will not be trapped between the lower end of the hammer piston 28 and the region between the upper portion of the anvil 18 and housing, which might tend to cause the piston to continue reciprocating in the housing 11, instead of the anvil and piston occupying the lower position illustrated in FIGS. 3a and 3b in which air can be caused to circulate through the apparatus, anvil 18 and bit C without afiecting operation of the apparatus.

A large torque transmitting surface between the segments 81 and the anvil 18 is provided. Thus, the torque transmission occurs between one of the side walls 309 of the drive member 16, through the surface 308 to each of the segments or keys 81, and from the convex surface 303 of such segment to the concave surface 304 of the anvil engaged therewith. During the transmission of the torque for the purpose of rotating the drill bit C during the operation of the air hammer, the anvil and bit can move longitudinally with respect. to the segments 81 and the housing 16, 11, but in view of the comparatively large area of the contacting surfaces between the segments 81 and the companion concave walls 304 of the anvil 18, wear between the parts is reduced. Despite such large area of contact, the crosssectional area of the anvil 18 and bit C throughout its length has been decreased to a substantially lesser extent than in torque transmitting arrangements embodying key and keyway spline types of connections. This is exemplified in the graph of FIG. 11, in which the cross-sectional area is plotted along the length of the anvil bit, beginning at its upper face 29 and extending downwardly to the drilling face 400 of the bit. Curve M shows the cross-sectional area ratio at each point along the length of the anvil bit of a prior drive connection embodying the key and keyway splines. Curve B shows the cross-sectional area ratio at each point along the length of the bit illustrated in FIGS. 1 to 6, from its upper end 29 to its lower end or bottom drilling face 400. It is evident that the cross-sectional area ratio of Curve B is greater than Curve M from the upper end 29 of the anvil to the bit shoulder 38 against which the lower end of the head or drive member 16 bears to transmit drilling weight therethrough. Thus, there is less change in cross-sectional area ratio throughout the length of the anvil and bit in Curve B than in Curve M, since the cross-sectional area ratio at the bit shoulder 38 is the same in both Curve M and in Curve B, asindicated by the points 401.

The result of a less change 'in cross-sectional area ratio occurring throughout the length of the anvil bit is a reduction in the loss due to reflections through the anvil bit of Curve B. By way of example, it can be shown that the loss due to reflections created by longitudinal stress pulses is approximately 5 percent in the anvil bit of Curve B; whereas, the loss through the prior anvil bit represented by Curve M is about 17 percent. Thus, the loss of energy transfer through applicants anvil bit, as represented by'Curve B, results in about a 12 percent improvement over the anvil bit represented by Curve M.

The drive mechanism illustrated in FIGS. 1 to 6, including the anvil bit represented therein, are more economical to manufacture than the prior key and keyway spline types of drives and anvil bits used in air hammers. Moreover, when wear does occur, it is relatively easy to replace the drive segments 81 without having to replace the entire drive member or sub 16, as in prior air hammers. As noted above, because of the large torque transmitting surface between the segments 81 and thee anvil bit 18, the wear is less in applicants drive mechanism and anvil bit than in prior drive mechanisms and anvil bits.

The drive mechanism and anvil bit illustrated in FIGS. 7 and 8 is similar to FIGS. 1 to 6, except that the drive segments 81 and anvil bit are of a different shape. Thus, the drive member 16 has openings 300a therein, such as circumferentially spaced openings, in which segments 01a are disposed, the exterior 8112 of the segments being curved and conforming to the wall of the surrounding housing 30 while the inner surface 81c of each segment is a chordal surface bearing against a companion chordal surface 81d formed on the anvil bit, such flat and chordal surface 810 of each segment engaging companion surfaces 300a defining the sides of the opening in which the segment is disposed. The top 310a and bottom 312a of each segment conform closely to the top and bottom of the opening 300a in the drive member; whereas, the segment itself is disposed within an elongate recess or groove 600 formed in the anvil bit, to permit the anvil and bit to drop downwardly to the position shown in FIG. 3b, in which air can circulate freely through the apparatus for discharge from the lower end of the bit, while preventing any air from being trapped between the anvil and housing (as in FIG. 3b, grooves 320).

The engagement between the inner surface 810 of each segment 81a and the companion surface did on the anvil bit is along a true chordal plane. A similar engagement is present between the curved segments 81 and the concave walls or surfaces 304 of the anvil bit 18, such as shown in FIG. 4. The essential structure and mode of operation and-the advantageous results achieved are substantially the same in both the FIG. 41 and the FIG. 8 embodiments, the coengaging drive surfaces 303, 304 and 81c, 81d in both embodiments being referred to herein as generally chordal surfaces; thatis, extending transversely across the anvil 13, as distinguished from a generally radial key and keyway spline type of connection of the prior art devices.

In FIGS. 9 and 10, the spline connection between the drive member portion 116 of the housing and anvil bit 1811 also includes chordal torque transmitting surfaces 81f, 81g between the drive member and the anvil 18b. Thus, the anvil has a portion of. extended length provided with one or more chordal or flat faces 81g on its periphery which engage companion chordal or flat faces 811 f on the drive member. The anvil 10b is inserted upwardly into the drive member 16, and a split, expandable ring 605 is then disposed over the upper end of the anvil, the drive member 16 then being threaded fully upwardly into the housing 30, the split ring engaging a downwardly facing housing shoulder 606. This ring projects inwardly of the inner surface 81f of the drive member, so that the upper head on the anvil will limit downward movement of the anvil 18b and bit C within the housing when the bit is elevated from the bottom E of the bore hole.

During the operation of the air hammer apparatus illustrated in FIGS. 9 and 10, the torque is transmitted from the inner chordal surfaces 81f of the drive member 16 to the anvil 18b. The cross-sectional area ratio is much greater along the length of the chordal surfaces 81g of the anvil than in prior devices, reducing the losses due to reflections created by the transmission of longitudinal stress pulses through the anvil and bit.

In the form of invention shown in FIG. I2, a single segment 700 is secured, as by welding material 701, to the drive member 16, this segment having a flat inner chordal face 702 engaging a companion chordal face 703 on the anvil 18c. The cross-sectional area through the anvil along the length of its chordal face is much greater than in prior devices, and, therefore, results in greater drilling efficiency in the air hammer apparatus. However, it is preferred to provide a balanced drive mechanism, such as two, three or four uniform, circumferentially spaced segments 700 engaging companion generally chordal surfaces 702 on the anvil.

We claim:

1. In an anvil bit adapted for use in the lower housing portion of percussion drilling hammer apparatus: a

body providing a lower bit portion adapted to engage a formation and an upper impact anvil portion adapted to be intermittently struck by a hammer piston of the hammer apparatus; said anvil portion having a cylindrical periphery adapted to be engaged by a companion cylindrical inner wall of the housing portion, said anvil portion further having circumferentially spaced external generally chordal surfaces extending inwardly from and interrupting said cylindrical periphery at spaced circumferential locations therearound, said chordal surfaces being adapted for surface engagement by driving means transmitting rotation and torque of the housing portion from the housing portion through said chordal surfaces to said anvil portion.

2. In an anvil bit as defined in claim 1; said external generally chordal surfaces being uniformly spaced from each other circumferentially of said body. I

3. In an anvil bit as defined in claim I; said generally chordal surfaces being elongate and extending parallel to the body axis. 4. In an anvil bit as-defined in claim 1; said external generally chordal surfaces being uniformly spaced from each other circumferentially of said body; said generally chordal surfaces being elongate and extending parallel to the body axis.

5. In an anvil bit as defined in claim 1; said generally chordal surfaces being planar.

6. In an anvil bit as defined in claim' ll; said generally chordal surfaces being concave.

7. In an anvil bit as defined in claim 1; said external generally chordal surfaces being uniformly spaced from each other circumferentially of said body; said generally chordal surfaces each being planar.

8. In an anvil bit as defined in claim 1; said external generally chordal surfaces being uniformly spaced from each other circumferentially of saidbody; said generally chordal surfaces each being concave.

9. In an anvil bit as defined in claim 1; said external generally chordal surfaces being uniformly spaced from each other circumferentially of said body; said generally chordal surfaces each being planar; said generally chordal surfaces being elongate and extending parallel to the body axis.

10. In an anvil bit as defined in claim 1; said external generally chordal surfaces being uniformly spaced from each other circumferentially of said body; said generally chordal surfaces each being concave; said generally chordal surfaces being elongate and extending parallel to the body axis.

11. In an anvil bit as defined in claim 1'; said lower bit portion having an upwardly facing shoulder extending laterally outwardly beyond said anvil portion and through which drilling weight imposed by the housing portion is adapted to be transmitted.

12. In an anvil bit as defined in claim 1; said external generally chordal surfaces being uniformly spaced from each other circumferentially of said body; said lower bit portion having an upwardly facing shoulder extending laterally outwardly beyond said anvil portion and through which drilling weight imposed by the housing portion is adapted to be transmitted.

13. In an anvil bit as defined in claim 1; said external generally chordal surfaces being uniformly spaced from each other circumferentially of said body; said generally chordal surfaces each being planar; said generally chordal surfaces being elongate and extending parallel to the body axis; said lower bit portion having an upwardly facing shoulder extending laterally outwardly beyond said anvil portion and through which drilling weight imposed by the housing portion is adapted to be transmitted.

14. In an anvil bit as defined in claim 1; said external generally chordal surfaces being uniformly spaced from each other circumferentially of said body; said generally chordal surfaces each being concave; said generally chordal surfaces being elongate and extending parallel to the body axis; said lower bit portion having an upwardly facing shoulder extending laterally outwardly beyond said anvil portion and through which drilling weight imposed by the housing portion is adapted to be transmitted. 

1. In an anvil bit adapted for use in the lower housing portion of percussion drilling hammer apparatus: a body providing a lower bit portion adapted to engage a formation and an upper impact anvil portion adapted to be intermittently struck by a hammer piston of the hammer apparatus; said anvil portion having a cylindrical periphery adapted to be engaged by a companion cylindrical inner wall of the housing portion, said anvil portion further having circumferentially spaced external generally chordal surfaces extending inwardly from and interrupting said cylindrical periphery at spaced circumferential locations therearound, said chordal surfaces being adapted for surface engagement by driving means transmitting rotation and torque of the housing portion from the housing portion through said chordal surfaces to said anvil portion.
 2. In an anvil bit as defined in claim 1; said external generally chordal surfaces being uniformly spaced from each other circumferentially of said body.
 3. In an anvil bit as defined in claim 1; said generally chordal surfaces being elongate and extending parallel to the body axis.
 4. In an anvil bit as defined in claim 1; said external generally chordal surfaces being uniformly spaced from each other circumferentially of said body; said generally chordal surfaces being elongate and extending parallel to the body axis.
 5. In an anvil bit as defined in claim 1; said generally chordal surfaces being planar.
 6. In an anvil bit as defined in claim 1; said generally chordal surfaces being concave.
 7. In an anvil bit as defined in claim 1; said external generally chordal surfaces being uniformly spaced from each other circumferentially of said body; said generally chordal surfaces each being planar.
 8. In an anvil bit as defined in claim 1; said external generally chordal surfaces being uniformly spaced from each other circumferentially of said body; said generally chordal surfaces each being concave.
 9. In an anvil bit as defined in claim 1; said external generally chordal surfaces being unifoRmly spaced from each other circumferentially of said body; said generally chordal surfaces each being planar; said generally chordal surfaces being elongate and extending parallel to the body axis.
 10. In an anvil bit as defined in claim 1; said external generally chordal surfaces being uniformly spaced from each other circumferentially of said body; said generally chordal surfaces each being concave; said generally chordal surfaces being elongate and extending parallel to the body axis.
 11. In an anvil bit as defined in claim 1; said lower bit portion having an upwardly facing shoulder extending laterally outwardly beyond said anvil portion and through which drilling weight imposed by the housing portion is adapted to be transmitted.
 12. In an anvil bit as defined in claim 1; said external generally chordal surfaces being uniformly spaced from each other circumferentially of said body; said lower bit portion having an upwardly facing shoulder extending laterally outwardly beyond said anvil portion and through which drilling weight imposed by the housing portion is adapted to be transmitted.
 13. In an anvil bit as defined in claim 1; said external generally chordal surfaces being uniformly spaced from each other circumferentially of said body; said generally chordal surfaces each being planar; said generally chordal surfaces being elongate and extending parallel to the body axis; said lower bit portion having an upwardly facing shoulder extending laterally outwardly beyond said anvil portion and through which drilling weight imposed by the housing portion is adapted to be transmitted.
 14. In an anvil bit as defined in claim 1; said external generally chordal surfaces being uniformly spaced from each other circumferentially of said body; said generally chordal surfaces each being concave; said generally chordal surfaces being elongate and extending parallel to the body axis; said lower bit portion having an upwardly facing shoulder extending laterally outwardly beyond said anvil portion and through which drilling weight imposed by the housing portion is adapted to be transmitted. 